It is known from the prior art various methods for increasing the power of a laser source. The most conventional method among these consists in using a doped-fiber amplification cascade, making it possible to highly increase the peak power Ppeak of the laser signal. Nevertheless, with such a method it is not possible to obtain a sufficient mean power Pmean for the aimed applications since it does not generally exceed a hundred Watts.
Another method consists in amplifying the laser signal in an optical resonator, or amplifying cavity, of the FABRY-PEROT type. In fact, in order to increase the number of photons produced through the COMPTON effect when the electron beam and the laser beam cross each other, it is necessary to reduce to the maximum the transverse and longitudinal dimensions of both beams. Nevertheless, in order to reduce the minimal transverse dimension of a laser beam within a FABRY-PEROT cavity composed of spherical mirrors, it is necessary that the distance between these two mirrors be as close as possible to twice the curvature radius of the mirrors.
Nevertheless, such a cavity being instable, it cannot be used in the frame of a real application. This is the reason why many devices for producing monochromatic X-rays generated through the COMPTON effect, such as those illustrated in OPTICS LETTER review, vol. 32, No. 19, Oct. 1, 2007, as well as in patent document No US 2008002813, use an amplifying cavity of the FABRY-PEROT type having four coplanar mirrors out of which two are spherical. The minimal transverse dimension may then be reduced of about tens of microns while exhibiting a good mechanical stability.
Meanwhile, the eigen states of polarization of a cavity with four coplanar mirrors have been calculated and it has been found that when the fineness of the cavity is high, that is, when the cavity gain is higher than about 1000, these eigen states vary strongly depending on inevitable mirror misalignments and vibrations. The polarization of the laser beam injected in the cavity being fixed, such variations lead not only to a variation of the intra-cavity polarization but also to an intra-cavity power variation. These power variations are detrimental in that the mean intra-cavity power Pmean is reduced.
Further, the existing devices also exhibit defects relating to the laser orientation precision that determines in part, the transverse dimensions of the laser beam. More particularly, the combination of mechanical parts used to orient the optical reflectors within the cavity and in vacuum experimental conditions lead to clearances that alter the cavity fineness.
Another problem raised by these systems relates to the use of motors in a vacuum medium. In fact, standard motors commercially available as being UHV compatible do not totally satisfy the requirements of ultrahigh vacuum such as required when using an optical resonator, in particular, owing to the high non pollution restrictions associated to the integration on the accelerator. In commercially available products, the hinges of the optical mounts are made with very weak or prestressed clearances, thus, greased to guarantee a precise positioning with no seizing risk. Two reasons exclude this type of hinging in vacuum conditions. On one hand, the use of grease is prohibited, and, on the other hand, between contact surfaces of a same nature, are generated microbonding phenomena which make their relative sliding impossible, particularly under vacuum conditions where the parts are very clean so as to satisfy the UHV conditions. These problems become even more essential as the number of motors required for the settings of the cavity considerably increases the level of pollution.